I think that sequence adds little to the game and should be removed.
It gives the AI 9% (0.95*(1-0.95*0.5)) chance at delaying Spanish colonisation. A Spanish player will never deny Columbus his journey, so why give the AI a chance to mess up? In MP a Portuguese player will either deny the Spanish player Columbus, or blackmail him for the sailor. Is that an improved situation?
I don't say that it's not plausibly that Columbus didn't work for Spain. But to REALLY simulate the REAL historical chance we must add up so many circumstances that contributed to make him end up where he did, so the final chances become very slim. (Why didn't he stay in Genoa?) Everything in history that's connected to certain persons had a very small chance of actually happening, but we should not try to simulate that. What were the chances Isabel and Ferdinand marrying, and the chances their family later marrying with the Habsburgs? There is no point in going into that amount of detail, since it virtually has no bottom!
I think we have to draw an arbitrary line somewhere regarding what things/flukes we should care to implement. They should improve the game, not give glaring advantages to the human player, not be exploitable and work well in MP. This sequence doesn't fullfill any of these things in my opinion.
The same is true to the question whether Spain should start colonising if they have taken out Granada. We give the AI big disadvantage since it has no chance of understanding the events beforehand. If Granada is conqered 1420, should we giva a whole host of probable explorers/conquistadors straight after that? No, I don't think so. But by making the fullfilled reconquista a prerequisite for colonisation we only pile up potential disadvatages to the AI, and no advantages. This is admittedly less of a clear case since there is historical justification for it, and no persons involved. But perhaps they would have sent explorers on the way even if Granada was still drawing it's last breaths? After all, the Portuguese were beginning to make a fair amount of profit from trading in new areas, Castile had already colonised the Canaries, the naval technology was there etc. It's not toatally unreasonable to let Spain explore, regardless of Granada. In my meaning we should, since it's hard to prove they would not have done something they actually did, and IT IMPROVES THE GAME!